Visualizing the role of applied voltage in non-metal electrocatalysts

ABSTRACT Understanding how applied voltage drives the electrocatalytic reaction at the nanoscale is a fundamental scientific problem, particularly in non-metallic electrocatalysts, due to their low intrinsic carrier concentration. Herein, using monolayer molybdenum disulfide (MoS2) as a model system of non-metallic catalyst, the potential drops across the basal plane of MoS2 (ΔVsem) and the electric double layer (ΔVedl) are decoupled quantitatively as a function of applied voltage through in-situ surface potential microscopy. We visualize the evolution of the band structure under liquid conditions and clarify the process of EF keeping moving deep into Ec, revealing the formation process of the electrolyte gating effect. Additionally, electron transfer (ET) imaging reveals that the basal plane exhibits high ET activity, consistent with the results of surface potential measurements. The potential-dependent behavior of kf and ns in the ET reaction are further decoupled based on the measurements of ΔVsem and ΔVedl. Comparing the ET and hydrogen evolution reaction imaging results suggests that the low electrocatalytic activity of the basal plane is mainly due to the absence of active sites, rather than its electron transfer ability. This study fills an experimental gap in exploring driving forces for electrocatalysis at the nanoscale and addresses the long-standing issue of the inability to decouple charge transfer from catalytic processes.


INTRODUCTION
Electrochemical reactions are a key strategy for realizing carbon neutrality and advancing the utilization of clean energy through the production of green hydrogen via water electrolysis with renewable electricity. Unraveling the electrocatalytic mechanism is of paramount importance to achieve a comprehensive understanding of the process and develop high-performance catalysts. Therefore, investigating the electrocatalytic mechanism holds significant promise for promoting a sustainable future. Applying voltage is an effective method for changing the electrochemical potential of an electrocatalyst [ 1 , 2 ], thereby determining the direction and rate of the reaction. However, the applied voltage (V appl ) regulates the electrochemical process differently for metal and non-metal catalysts. For metal catalysts with a huge density of states (DOS) near the Fermi levels (E F ), the potential mainly drops within the electric double layer (EDL), modulating the electrostatic potential ( ϕ) and changing the reaction coordinates directly [ 1 ]. While for non-metallic catalysts, due to the low electrical conductivity and complex changes in chemical composition, the potential distribution across the solid-liquid interface becomes complicated [ 3 -8 ]. A part of the potential drop may drop across the non-metal lic catalysts, whi le another part of the potential drops across the EDL [ 6 -8 ]. In this case, both the chemical potential ( μ e ) and the electrostatic potential ( ϕ) are changed, resulting in the complex electrocatalytic mechanism. Due to the wide variety of non-metal electrocatalysts, finding the universal rules is a great challenge. For example, it was reported that electrocatalytic oxygen evolution reaction (OER) is driven by the chemical potential ( μ e ) in an IrO x catalyst, rather than its electrostatic potential [ 2 , 9 ]. While for M-N-doped carbon catalysts, the electrocatalytic reaction is driven predominantly by the change in electrostatic potential across the EDL [ 2 , 10 , 11 ]. If the potential distribution across the solid-liquid interface can be directly measured, it wi l l be very convenient for studying the electrocatalytic mechanism of non-metallic catalysts.
Up until now, distinguishing the potential distribution between the non-metallic catalyst and the EDL has sti l l relied on complex theoretical calculations [ 3 , 4 , 12 ]. By using empirical formulas of capacitance, Bediako et al. showed that the potential distribution between twisted bilayer graphene and EDL is a function of V appl [ 8 ]. In the field of conventional electrochemical technology, Mott-Schottky analysis is a common method for calculating the flat band potential of semiconductors. However, it cannot provide information on how the band structure moves, which is related to the interfacial potential distribution. Through I-V curve, the onset potential of a low-DOS semiconductor at which the E F reaches the band edge can be obtained [ 13 ]. However, the potential drop across the semiconductor-electrolyte interface sti l l remains unknown, and hence it is not possible to obtain the respective shift of E c and E F . The value of the total interface capacitance ( C i ) can be obtained by electrochemical impedance spectroscopy (EIS). However, the specific capacitance of the non-metallic electrode and EDL cannot be directly measured by EIS, which requires calculations based on the non-metallic structure model and the double layer (Gouy-Chapman-Stern) model, respectively [ 6 ]. In summary, there is a lack of in-situ techniques for directly measuring the potential distribution between the non-metallic catalyst and the EDL. Moreover, conventional electrochemical characterization only provides the ensemble information for electrode materials, neglecting the spatial heterogeneity in the electronic structures of catalysts [ 14 -20 ]. Therefore, a spatially resolved in-situ characterization technique is needed.
Scanning electrochemical potential microscopy (SECPM), invented by Bard's group, has been used to measure EDL profiles on electrode surfaces, demonstrating the feasibility of scanning probe techniques for potential measurements in liquid [ 21 ]. Boettcher et al. developed a contact-based potentialsensing electrochemical atomic force microscopy technique and successfully mapped the photovoltage at nanoscopic semiconductor-catalyst interfaces [ 22 , 23 ]. This shows that changes in surface potential of catalysts can be sensed through contact mode. Based on these pioneering works, we focus on the key issues in electrocatalysis, to reveal the working principle of electrocatalysts through insitu surface potential microscopy. Recently, the spatial heterogeneity of local electrostatic potential at Au nanoplate-solution interfaces was revealed under open circuit potential (OCP) conditions [ 24 ]. However, it is sti l l a chal lenge to figure out how the electronic structure of non-metallic electrocatalysts changes under applied voltage.
Semiconductors have been predicted to be nonideal catalysts due to their low intrinsic carrier concentration [ 25 ]. For example, it is widely believed that the basal plane of MoS 2 impedes charge transfer in electrocatalytic reactions [ 26 -31 ]. According to classical electron transfer theories, the Schottkyanalogue junction is broken and becomes ohmic once the E F is tuned into the bands, resulting in the low conductivity of the semiconductor in liquid [ 13 , 32 ]. However, it is difficult to explain the high electrocatalytic activities of some semiconductor catalysts [ 13 , 33 , 34 ]. Liu et al. found that the ultrathin MoS 2 can be modulated to be highly conductive ('on') or insulating ('off'), strongly correlating with the hydrogen evolution reaction (HER) [ 13 ]. Morpurgo's group also observed liquid-gating-induced superconductivity in thin MoS 2 crystals [ 35 ]. The Y 1.75 Co 0.25 Ru 2 O 7 −δ electrocatalyst was found to enhance the charge transfer step in the oxygen evolution reaction (OER) [ 36 ], while the β-Co(OH) 2 electrocatalyst exhibited gradually increasing conductivity after the onset potential of the OER [ 17 ]. The above results showed that a charge transport pathway can be open under solution conditions, reflecting the dramatic change of the energy band structure of the semiconductor. However, the underlying reason for the enhanced conductance is not clear.
MoS 2 is a representative material in both the fields of electrocatalysis and semiconductor devices. Studying how the voltage affects the band structure and conductivity of MoS 2 under solution conditions is of great research significance. Herein, using MoS 2 as a non-metallic model system, the potential distribution between the basal plane of monolayer (ML) MoS 2 ( V sem ) and the electric double layer ( V edl ) were quantificationally decoupled as a function of the applied voltage. We visualized how E F moves into E c of the basal plane of MoS 2 , thereby realizing the transition of conductivity, which is the origin of the enhanced conductance in liquid. We provide a method for studying electrochemical driving force at the nanoscale and demonstrate that only when the electron transfer site and the chemical site are spatially coincident can the energy conversion efficiency be maximized. This work has significant importance in advancing our understanding of the electrocatalytic mechanism, and provides theoretical guidance for the conversion of clean energy and the achievement of carbon neutrality.

Decoupling the potential drops across the semiconductor ( V sem ) and the electric double layer ( V edl )
Schematic i l lustrations of the home-built in-situ surface potential microscope are shown in Fig. 1 A and Fig. S1. By combining a high impedance amplifier (1 T ) to the atomic force microscopy (AFM) positioning, the local surface potential V s of the electrode can be obtained. This is achieved by reading the potential of the tip (V tip ) in direct contact with the electrode surface relative to the A g/A gCl reference based on an assumption: the E F of the tip is controlled by the solution and not by the semiconductor in the ranges of applied voltage (the tip and the semiconductor are difficult to equilibrate, so the E F of the tip does not follow the E F of the semiconductor, and the potential of the tip (V tip ) reflects the EDL potential at the surface of the semiconductor (V s )).
We take V s (V appl = 0) as a reference. The change of potential drop over the EDL ( V edl ) can be read directly by V s (V appl = 0) −V s (V appl = 0). By combining with the change of applied voltage ( V appl ), the change of potential drop across the electrocatalyst ( V sem ) can also be obtained by V sem = V appl − V edl . To eliminate the effect of redox reactions on the tip surface, which affect the potential measurements, we used a solution that contains only the supporting electrolytes (0.1 M K 2 SO 4 ) and studied the intrinsic properties of ML MoS 2 at different V appl .
Due to the small Debye screening length, the distance between the tip and the sample determines whether an accurate surface potential can be measured. To demonstrate the feasibility of surface potential measurement, a metal (Au) substrate was used as a standard sample. The tip was rested on the surface with an applied force of ∼29 nN, slightly larger than that reported by Boettcher et al.
( ∼25 nN) [ 21 ] but sufficient to prevent damage to the sample. As shown in Fig. 1 B, V s faithfully tracked V appl , reflecting that the surface potential V s is equal to the applied voltage. This is consistent with the common view for bulk metal electrodes [ 1 , 37 ] that there is no potential drop inside the metal. In this situation, the surface electron concentration n s can be considered a constant value and V appl is applied to increase the electric field intensity of EDL ( V appl = V edl ), as indicated by curve III of Fig. 1 A. With the same applied force (29 nN), the tip can be considered to be able to contact the surface of other samples. Then, the tip was lifted off ∼1 mm from the surface and the potential of the solution (background) was measured ( Fig. 1 B). It shows that the change in solution potential ( ∼12 mV) is negligible because the thickness of the EDL is less than 1 nm in a 0.1 M K 2 SO 4 solution [ 38 ]. Therefore, the solution can be considered a blank background and does not affect the surface potential measurement.
Next, the surface potential of the basal plane of ML MoS 2 at different V appl was monitored, as shown in Fig. 1 B. Under OCP conditions (0.14 V), there was no apparent V s change when the tip was transferred from the solution to the surface of MoS 2 . The result reflects that the solution has equilibrated the E F of the tip and MoS 2 before contact and the electron transfer between the tip and MoS 2 can be ignored. The result demonstrates that the introduction of the nanotip did not change the initial structure of MoS 2 . When V appl > −0.15 V (vs. A g/A gCl), there is no change in V s . V s starts to track V appl when V appl < −0.15 V. The change in EDL potential ( V edl ) as a function of applied potential can be obtained by V s (V appl = 0) −V s (V appl = 0), as shown in Fig. 2 A (blue circuit). Since a part of the potential Natl Sci Rev , 2023, Vol. 10, nwad166 shifts the E F of the basal plane by −e V appl relative to its initial position. The potential change in EDL ( V edl ) shifts the full band structure (including the conduction band (E c ) and the valence band (E v )) by −e V edl relative to its initial position. The potential change in the semiconductor ( V sem ) corresponds to the relative movement rate between E F and E c , shifting the E F by −e V sem with respect to the E c , which contributes to the change of surface conductance. drop over EDL is obtained, another part of the potential drop across the basal plane of MoS 2 can also be given by V sem = V appl − V edl (red circuit, Fig. 2 A). The results clearly show that when V appl > −0.15 V, the total potential is dropped across the ML MoS 2 ( V appl = V sem ), as shown in Fig. 1 A, potential curve, type I, while the surface potential remains constant. When V appl is below −0.15 V, the surface potential begins to change, resulting in a partial drop of the potential across the solution ( V appl = V sem + V edl , Fig. 1 A, potential curve, type II). V edl increases as V appl becomes more negative. Af-ter decoupling the potential drops in the basal plane ( V sem ) and the EDL ( V edl ), the evolution of the band structure under electrode conditions can be further revealed.

The evolution of the band structure under electrode conditions
The evolution of the potential curve t ype I to t ype II reflects a dramatic change in band structure of the basal plane. As shown in Fig. 2 B, the change in applied voltage ( V appl ) shifts E F of the basal plane by −e V appl relative to its initial position [ 4 , 8 ], which is considered as the movement rate of E F in the energy coordinates. The potential change in EDL ( V edl ) shifts the full band structure (including the conduction band (E c ) and the valence band (E v )) by −e V edl relative to its initial position [ 1 ], which can be seen as the movement rate of E c . The potential change in the semiconductor ( V sem ) corresponds to the relative rate difference between E F and E c , shifting E F by −e V sem with respect to E c , which contributes to the change in surface conductance or n s [ 4 , 8 ]. The potential distribution between the catalyst ( V sem ) and the electrolyte ( V edl ) is determined by the series of semiconductor capacitors (C sem ) and EDL capacitors (C edl ) as shown in Fig. 1 A (bottom). Additionally, due to the smaller DOS of 2D semiconductors compared to bulk semiconductors, 2D semiconductor capacitors function as quantum capacitors (Cq), where 1/C tot = 1/C q + 1/C edl [5 6 , 39 ]. When V appl > −0.15 V, V appl = − E F /e and V edl = − E c /e = 0. V sem = V appl , indicating that C q C edl (that is, the E F is located in the band gap with zero DOS near the E F ). In this case, E c is pinned at a fixed value in the energy coordinates, and E F moves to the band edge of E c at the maximum rate ( Fig. 2 C, iv, iii and ⅱ ). However, the transition from t ype I to t ype II potential drop occurs at an onset potential of approximately −0.15 V, indicating that the E F is tuned into the bottom of the E c at V appl = −0.15 V (Fig. 2 C, ⅱ ). This results in an increase in C q in the basal plane of ML MoS 2 due to the large DOS near E F , as shown in the top panel of Fig. 2 B. When V appl < −0.15 V, V appl = − E F /e, V edl = − E c /e and V sem = V appl − V edl . The E c starts to move. As V appl becomes more negative, E c moves faster. In this situation, the relative rate between E F and E c gradually decreases, as shown in Fig. 2 C, i. However, the difference in the movement rate between E F and E c sti l l exists, which means that the Fermi level keeps moving into the conduction band. These results are inconsistent with the classical theory that the Schottky-analogue junction is broken once the E F is tuned into the conduction band, and it cannot be further tuned deep in bands [ 32 ]. The bandgap E gap = 1.85 eV of ML MoS 2 can be determined from photoluminescence spectroscopy (PL), depicted in Fig. S10C. Since the energy difference between E F and E c determines the surface charge concentration (n s ), for 2D semiconductor materials, n s can be calculated by (see Supplementary Data for derivation details): where N c ∼8.6 × 10 12 cm −2 is the effective density of states in the conduction band [ 40 ]. The conductivity of the basal plane is constantly changing since the relative positions of E F and E c in the energy coordinates change with the V appl . Figure 2 D plots the relationship between n s and V appl . The n s -V appl curve shows the switching effect of n s : the surface is turned 'on' with high electron concentration (over 1 × 10 13 e cm −2 in the basal plane of ML MoS 2 , consistent with the 'liquid gating' effect [ 13 , 35 , 41 , 42 ]) at V appl below −0.15 V, and is turned 'off' with low electron concentration (insulating) V appl higher than −0.15 V. The results show that the surface electron concentration can be effectively tuned under electrolyte conditions. Through the in-situ surface potential measurement, we visualized the evolution of the band structure under electrode conditions and how the high conductivity on the semiconductor surface occurs. Moreover, it can be seen from the n s -V appl curve that the basal plane of ML MoS 2 is nearly insulated at the open circuit potential ( ∼0.14 V). It corresponds to the depleted state of ML MoS 2 with low electron concentration in solution (the flat band potential is approximately equal to −0.07 V, estimated from a macroscopical Mott-Schottky measurement, Fig. S9).
The switching effect of n s in the basal plane was further proven by conductivity measurements [ 42 -44 ]. An in-situ local electric conductivity measurement was performed, shown in the inset of Fig. 2 D. With the tip directly contacting the surface of the basal plane (lift = 0, substrate generation/tip collection (SG/TC) mode), a linear sweep voltammetry (LSV) curve was collected on the substrate while the potential of the tip remained constant. In this way, the conducting current I is recorded at the solid-liquid interface (containing only the supporting electrolytes to exclude the Faradic reaction). It is shown that the basal plane of ML MoS 2 becomes conductive ('on') once the V appl is below the onset potential of approximately −0.15 V. Otherwise, the basal plane maintains an insulating state ('off'). In previous studies, the onset potential of the conductivity of low DOS semiconductor materials (2D WS 2 , MoS 2 , etc.) was regarded as the potential at which E F reaches the position of the band edge of E c [ 13 , 45 ], proving the accuracy of the surface potential measurement. To date, how the band structure of the semiconductor is transformed under electrolyte conditions has not been clearly studied by conventional electrochemical methods. The difficulty is in distinguishing the potential distribution between semiconductor and EDL, which sti l l depends on theoretical calculations. In our study, the potential drops in the semiconductor ( V sem ) and the EDL ( V edl ) are decoupled by direct measurements. Based on the potential values, the evolution of band structure can be revealed under electrolyte conditions.

The role of the applied voltage in electrocatalytic reactions
Electron transfer (ET) and the subsequent formation and rupture of chemical bonds (catalytic reaction) are two fundamental processes in electrocatalytic reactions [ 9 ]. However, it is difficult to distinguish their contributions from each other due to their convoluted nature. It becomes even more challenging for non-metal electrocatalysts with spatial heterogeneity in electronic structures and catalytic centers [ 46 , 47 ].
To study how V appl acts in an electrocatalytic reaction, first we used outer-sphere redox pairs to decouple the ET process from the electrocatalysis process and figure out how the V appl drives the ET process. Atomic-force-microscope-based scanning electro-chemical microscopy (AFM-SECM) [ 24 , 48 , 49 ] (Fig. S11) was used to map the local electrochemical activity of ML MoS 2 electrocatalysts in SG/TC mode [ 50 , 51 ]. A higher i Tip reflects a higher local product concentration (that is, higher local electrochemical activity) of the electrocatalysts. The outersphere redox pairs with the reaction [Ru(NH 3 ) 6 ] 3 + + e − → [Ru(NH 3 ) 6 ] 2 + were used to provide information about the ET [ 20 ]. The ET image clearly shows that the basal plane of MoS 2 has comparable ET activity to that of the edge sites (Fig. 3 A). The ET currents in the back of the topographic image are a bit higher than those in the front, due to the change in local concentration of redox pairs near the surface during the imaging process (scanning from back to front in Fig. 3 A). Histograms of ET current distributions at different sites are shown in Fig. 3 B. The ET currents taken from the single scan line collected in a short time are comparable. Figure 3 C gives the topography and ET current values obtained along the same scan line (the dashed line in Fig. 3 A). The current values of the basal plane and the edge differ by only 15 pA. The spatially resolved LSV curve of the basal plane largely matches that of the edge (Fig. 3 D), indicating no significant difference in the ET rate between basal plane and edges at different biases. ET imaging shows high ET activity of the basal plane, contrary to the traditional consensus under ex-situ conditions [ 52 , 53 ]. It is reasonable to speculate that the high ET activity may derive from the aforementioned 'liquid gating' effect with high n s . However, ET currents are jointly decided by both the n s and rate constant k f ( i ∝ n s and k f ) [ 1 ], that is where F is Faraday constant, A is electrode area, n s is surface electron concentration related to surface electrical conductivity, and C 0 is the reactant ([Ru(NH 3 ) 6 ]Cl 3 ) concentration near the surface of the electrode. Here, the reverse reaction is negligible. The rate constant k f can be expressed as k 0 ·exp[ −αf(E s -E 0' )], in which k 0 is the standard rate constant, E s is surface potentials of the electrode, α is the transfer coefficient, f = F RT , and E 0' is the formal potential. In a solution containing 5 mM [Ru(NH 3 ) 6 ] 3 + , the formal potential E 0 , estimated as E 1/2 of the metal substrate, is −0.14 V for [Ru(NH 3 ) 6 ] 3 + /2 + (Fig. S12B), which is close to the V appl at which MoS 2 becomes semi-metallic. Only when the voltage is applied to change the n s , rather than k f for the basal plane, can the 'liquid gating' effect be responsible for the high ET activity. To date, it is sti l l unclear how much the n s contributes to the ET rate in non-metal electrocatalysts.
Herein, the electronic structure of the basal plane measured in supporting electrolytes (without Faraday currents) was used to predict the factors affecting the ET activity and give a meaningful reference. According to k f = k 0 ·exp[ −αf(E s -E 0 )], since k 0 is unknown it is hard to obtain the specific value of k f . However, k f at different V appl can be compared by k f (E s,1 )/k f (E s,2 ) = exp[ −αf(E s,1 -E s,2 )]. Taking k f at V appl = 0.4 V as the reference point (marked as k f,0 ), we can obtain k f /k f,0 at different V appl . Similarly, we can also have n s /n s,0 at different V appl (the reference point n s,0 is n s at V appl = 0.4 V). Combining k f /k f,0 and n s /n s,0 , the potential-dependent behavior of k f and n s can be obtained (Fig. 3 E). As shown in Fig. 3 F, when V appl is more positive than the formal potential E 0 (V appl > −0.14 V), the electrochemical potential (or E F ) of MoS 2 is lower than that of the [Ru(NH 3 ) 6 ] 3 + /2 + redox pair and there is no driving force to transfer electrons from MoS 2 to the solution (ET reaction does not occur). In this voltage range, all potential drops within the semiconductor increase the n s rapidly with a |Slope| ≈ 17 for log(n s /n s,0 ). There are no potential drops within the EDL, so the electric field intensity of EDL remains constant and k f does not change with |Slope| ≈ 0 for log(k f /k f,0 ). In this voltage range, the effect of the applied voltage can be considered as a 'preparation' for the ET reaction, transforming MoS 2 from a semiconductor state to a semi-metallic state. When V appl is more negative than the formal potential E 0 (V appl < −0.14 V), the electrochemical potential of MoS 2 is higher than the [Ru(NH 3 ) 6 ] 3 + /2 + redox pair, creating a driving force for electrons to transfer from MoS 2 to solution, and the ET reaction begins to occur. In this voltage range, the potential drop within the EDL gradually increases. The V appl begins to act on the electric field intensity of EDL, changing the value of k f . When V appl < −0.3 V, the V appl increases k f rapidly with |Slope| ≈ 6 for log( k f /k f,0 ), whi le the V appl increases n s slowly with |Slope| ≈ 1 for log(n s /n s,0 ). When the ET reaction occurs, the effect of the applied voltage is to a greater extent to change the k f value, prompting electrons to pass through the solid-liquid interface for the ET reaction.
The performance of the ML MoS 2 in an electrocatalytic (inner-sphere) reaction was also investigated at the nanoscale. Figure 4 A displays HER current mapping superimposed on a 3D topography map. The result shows that the high tip current ( i Tip ) is collected at the edge sites, while the current at the basal plane is weak. Histograms of HER current distribution at different sites are shown in Fig. 4 B. Compared with the outer-sphere reaction, the inner-sphere reaction is more sensitive to the chemical properties of the active site. The HER currents at different edge sites vary greatly, ranging from a few pA to hundreds of pA. The differences in HER currents reflect the variation in the chemical properties of edge sites under in-situ conditions. Figure 4 C gives four current-topography plots along the dashed lines in Fig. 4 A, representing the edge sites with different HER activities. Despite differences in the current, the topography of the edge corresponds exactly to the maximum of the HER current. In addition, the spatially resolved LSV method was also performed for ML MoS 2 (Fig. 4 D). The basal plane is almost catalytically inert over a wide potential range, while the electrochemical currents at the edge increase exponentially with increasing negative bias. In a previous study, Norskov et al. found that the HER activity of MoS 2 is proportional to its edge length, and thus hypothesized that the edges are the active sites of the HER [ 15 ]. Our work provides direct evidence for the structure-activity relationship of MoS 2 through in-situ HER imaging. With the outer-sphere and the inner-sphere reaction, the spatial mismatch of charge transfer and chemical reaction processes of ML MoS 2 was clearly revealed at the nanoscale. The applied voltage plays an essential role in the electron transfer step, increasing the surface electron concentration n s and the rate constant k f of the basal plane. However, the large number of electrons reaching the surface cannot participate in the chemical reactions (Fig. 4 E). The large Gibbs free energy of the atomic hydrogen ( G H* ) adsorbed on the basal plane (1.96 eV) prevents chemical processes at the solid-electrolyte interface (Fig. 4 F). Therefore, the low binding energy of H contributes to low HER rates through the effect of high barriers, resulting in low energy-conversion efficiency.

CONCLUSIONS
Through in-situ surface potential measurements, the potential distribution between the basal plane of ML MoS 2 ( V ch ) and electrolyte ( V edl ) was decoupled quantitatively as a function of the applied voltage. We visualized the evolution of the band structure under electrode conditions and revealed the process of conductivity transformation on the semiconductor surface. To clarify how the applied voltage acts on the electrocatalytic reaction, the ET and catalytic reaction processes on ML MoS 2 were identified at the nanoscale through AFM-SECM mapping. We clearly showed that the applied voltage plays an essential role in the ET process of the basal plane of MoS 2 , increasing the surface electron concentration n s and the rate constant k f to different degrees under different V appl . However, the applied voltage cannot be precisely applied to the electrocatalytic reaction due to the spatial mismatch of charge transfer and reactant adsorption sites. This work paves the way for the rational design of efficient non-metallic electrocatalysts based on the understanding of how voltage acts on non-metallic catalysts at the nanoscale.

SUPPLEMENTARY DATA
Supplementary data are available at NSR online.